AUV3Dm: Detailed Characterization of Shallow Soil Strata and Geohazards Using AUV Subbottom Profiler 3D Micro Volumes

Abstract

Abstract This paper describes the acquisition, processing, and application of deepwater subbottom profiler data acquired using an AUV survey vehicle and processed as a micro 3-D seismic data volume (AUV3Dm data). AUV3Dm data precisely defines foundation-zone conditions in 3-D space without gaps. The AUV3Dm methodology consists of doing a very detailed 3-D seismic survey (4-m line spacing) over a very small area (for example, 390 × 500 meters) using a high-frequency seismic (sub-bottom profiler, SBP) source (typ. 2 kHz to 12 kHz). The resulting 3-D data volume has a bin spacing of 4 × 0.5 meter, provides near-core-scale vertical resolution to a maximum of 75 to 100 m or so below seafloor, and allows the robust data-analysis functionality inherent in 3-D seismic volumes (arbitrary lines, attribute extractions, time and horizon slices, horizon flattening, etc.). This methodology is being used where a very detailed 3-dimensional characterization of shallow soil strata and geologic features (such as thin, discontinuous sandy zones; fault planes; fluid vents; gassy zones, etc.) is required to help optimize deepwater production structure siting and foundation design. One specific application is providing detailed characterization of proposed anchor-pile sites so that faults and other potential hazards can be avoided when installing the anchors. Further, the ~20-cm vertical resolution of the SBP data facilitates detailed correlation with sediment cores and cone-penetrometer data, thus allowing " calibration?? of the micro 3-D volume with geotechnical properties, and enhancing its use as a tool to predict soil conditions away from geotechnical stations. Use of the AUV3Dm methodology should be considered whenever shallow foundations are required in areas of complex stratigraphy or local geologic hazards. Introduction This paper describes the acquisition, processing, and application of deepwater subbottom profiler data acquired using an AUV survey vehicle and processed as a 3-D seismic data volume (AUV3Dm data). The AUV3Dm acronym is derived from: " Autonomous Underwater Vehicle (sub-bottom profiler) 3-D micro?? methodology/survey/data. The AUV3Dm methodology was first reported on for deepwater oil industry applications in 2005 (Campbell and others, 2005). This paper is an update to the original 2005 paper and reports significant improvements in this still-evolving methodology. We also include a number of data examples and images extracted from a recently acquired AUV3Dm volume to help illustrate the major improvements that have been achieved in data quality, and the resulting benefits provided by AUV3Dm data. An AUV3Dm survey consists of doing a very detailed 3-D seismic survey over a very small area using a high-frequency seismic (sub-bottom profiler, SBP) source operated from an AUV survey platform. For the AUV3Dm example survey described here, the 3-D survey area was about 390 × 500 meters, line spacing was 4 meters, and a 2 to 12 kHz Chirp SBP was the source. The resulting 3-D seismic volume has a bin spacing of 4 × 0.5 meters. Effective penetration below the seafloor was about 40 meters in this case, but penetration of as much as 75 to 100 meters is possible in areas of favorable geology. AUV3Dm data precisely defines foundation-zone conditions in 3-D space without gaps, provides near-outcrop-scale information as a 3-D seismic volume, and gives virtually the same benefits and robust data-analysis functionality inherent in 3-D seismic volumes. Thus, AUV3Dm data provides a much more reliable solution than does the 2-D data acquired during a conventional deepwater site survey that typically results in data gaps of 100 to several hundred meters between survey lines.